The uptake and export of silicon and nitrogen in HNLC waters of the NE Pacific Ocean

The high-nitrate, low-chlorophyll (HNLC) waters of the Gulf of Alaska tend towards silicate rather than nitrate depletion as phytoplankton utilize nutrients during summer. This tendency is enhanced when iron supply is elevated through natural inputs such as from coastally generated mesoscale eddies or through artificial enrichment as was carried out in an in situ experiment in July 2002. However, ship-board incubations with iron enrichment demonstrate nitrate rather than silicate depletion for these waters. The difference between in situ and in vitro experiments occurs at least in part because deck incubations do not allow export of particulate Si and N. Due to the more efficient recycling of nitrogen and carbon, export favours the removal of silicon from the upper ocean (the Si pump). Previous measurements at Ocean Station Papa (50°N, 145°W) show that ∼25% of the Si, but only ∼7% of the C and ∼4% of the N utilized during spring growth, is exported to a depth of 200 m. These results in the Gulf of Alaska agree with the present understanding of phytoplankton controls in other HNLC regions and show that any estimates of carbon export from iron enrichment should be based on Si- rather than N-limitation.